It simply means the web server was unable to understand the request of the client and process it.
It's a message saying that your internet browser has found something malformed or illegal in what you searched.
Please find below an article I have written on my blog to solve that issue: "How to Fix the "400 Bad Request" error message from a website" http://wp.me/p2519N-cB
400 Bad Request
Bad disc or bad radio.
A 201 error message appears which indicates the RAM failure.
A couple things could be wrong...... first thing you always want to try is another key, (if you have one) then if the car starts and no error message you know you have a bad key. if you still have a no start and error message it might be the transponder coil located on the ignition switch. could also be a bad imobilizer. Question.....does the engine start and then die right away? Or does the engine not start at all? a bad transponder coil will usually cause the start then die complaint. good luck!
Try going to a site that has answers instead of questions. That's what I am about to do.
In 1992, Tim Berners-Lee defined all the HTTP (Hypertext Transfer Protocol) codes based on the FTP (File Transfer Protocol) status codes. If you break it down, the first 4 indicates the error is a client error, the 0 indicates a general syntax error, and the final 4 refers to the specific error code in the 4xx group of error codes. In total there are 17 error codes in the 4xx group.
Missing or corrupt boot sector
When you try to boot the system, you will get a POST error message and will not be able to boot.
The 17 Codes in the 4xx Group are: 400 - Bad Request 401 - Unauthorized 402 - Payment Required 403 - Forbidden 404 - File Not Found 405 - Method Not Allowed 406 - Not Acceptable 407 - Proxy Authentication Required 408 - Request Timeout 409 - Conflict 410 - Gone 411 - Length Required 412 - Precondition Failed 413 - Request Entity Too Large 414 - Request-URL Too Long 415 - Unsupported Media Type 416 - Requested Range Not Satisfiable 417 - Expectation Failed
Wen says bad request the server isn't offline, you can change it in server.properties.